If an ifa has as NULL ifp pointer then is should be considered as

If an ifa has as NULL ifp pointer then is should be considered as
invalid.  When such thing happens, it means that the address is no
longer configured on the system but still referenced by some routes.
So do not return such ifa in ifa_ifwithroute().

Fix a panic reported by Pierre Bardou.

ok mikeb@, henning@